Is Helmholtz Resonance a Problem for Micro-Jet Actuators?

Numerical-simulation studies are undertaken to investigate how Helmholtz resonance affects the interaction of nominally inactive micro-jet actuators with a laminar boundary layer. Two sets of numerical simulations are carried out. The first set models the response of an actuator in ambient conditions to a small jump in its internal pressure. These results verify our theoretical criterion for Helmholtz resonance. In the second set of simulations, two-dimensional Tollmien-Schlichting waves, with frequency comparable with, but not particularly close to, the Helmholtz resonant frequency, are incident on a nominally inactive micro-jet actuator. The simulations show that under these circumstances the actuator acts as a strong source of 3D Tollmien-Schlichting waves. It is surmised that in the real-life aeronautical applications with turbulent boundary layers, broadband disturbances of the pressure field would cause nominally inactive actuators to act as strong disturbance sources. Should this be true, it would probably be disastrous for engineering applications of such massless microjet actuators for flow control.